Water temperatures – Big differences within the lakes – Meteo

For a relaxing swim in the lake, it helps to be in the right place at the right time. While the water temperature in Biel, for example, climbs just over 16 degrees in the afternoon, you can swim in the lake at the other end of the lake in Le Landeron at a pleasant 19 degrees.

As most swimming fans already know, the water temperatures in the lakes are highest towards the evening. The sun warms up the top layer of water during the day. This daily fluctuation is usually 1 to 2 degrees, which can already make a decisive difference at the current temperatures of around 18 degrees.

Place must be well chosen

In addition to the time of day, the location is also important. When large streams and rivers from the foothills of the Alps and the Alps flow into a lake, it is significantly cooler there. There are large temperature contrasts, for example, in Lake Zurich between the Obersee east of the Seedamm and the Untersee west of it. The Obersee is up to 5 degrees cooler than the Untersee due to the inflow of cold water from the Linth and the Wägitaler Aa.

Legend:

The large temperature differences in Lake Zurich are mainly due to the inflow of cold water.

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Wind as a spoilsport

When strong winds blow across a lake from a constant direction, the warm surface water is blown away. To compensate, much colder water flows in from below. There was an extreme example of this at the beginning of July 2017 on Lake Geneva. In the Geneva lido, temperatures dropped by 15 degrees within a few days due to a constant south-west wind.

Legend:

Modeled surface temperature of Lake Biel for June 7 at 6 p.m. The Bise ensures that the water temperature in Biel is significantly lower than in Le Landeron.

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The deeper the colder

In the summer months, the surface water is heated by the sun. The deeper you dive, the cooler it gets. Incidentally, in deep lakes the water temperature at the bottom is always 4 degrees. Because the water is densest at this temperature, it forms a relatively uniform layer at the bottom of lakes that is decoupled from the surface water.